Method for manufacturing a constructed camshaft

ABSTRACT

The invention relates to a method for manufacturing a constructed camshaft starting from a tube into which a plurality of noncircumferential depressions, which are oriented parallel to the longitudinal axis and are transverse with respect to the longitudinal axis, are pressed, and separately prefabricated cams are attached to the tube. In this context, the pressing-in of the depressions and the attachment of the cam rings to the tube take place in one working step.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of international patent application PCT/EP2004/002236 filed Mar. 3, 2004 and designating the U.S., which was not published under PCT Article 21(2) in English, and claims priority of German patent application DE 103 13 812.9, filed Mar. 21, 2003, which is incorporated herein by reference.

FIELD OF THE INVENTION

The invention relates to a method for manufacturing a constructed camshaft starting from a tube into which a plurality of noncircumferential depressions, which are oriented parallel to the longitudinal axis and are transverse with respect to the longitudinal axis, are pressed, and separated prefabricated cams are attached to the tube.

BACKGROUND OF THE INVENTION

Camshafts are used for controlling valves in internal combustion engines of motor vehicles. As technical progress has continued these internal combustion engines have been constructed with increasingly compact designs. The fabrication of engines, specifically the assembly, takes place in a highly automated fashion so that in this context use is increasingly being made of easy-to-assemble modules which are pre-assembled by the system suppliers. For reasons of space, the camshafts are often positioned in pairs so close to one another that direct access to the cylinder head bolts is no longer possible. As a result of these requirements, there is a need for specially shaped camshafts which allow, for example, automatic screwdriver devices to access the cylinder head bolts.

In relation to constructed camshafts composed of a tube and prefabricated cams attached thereto, DE 20116112 UI specifies a solution with which access to the cylinder head bolts is made possible in the installed state of the camshaft via depressions formed in the tube.

When such a camshaft is manufactured, the desired depressions are produced on the initial tube by pressing a plurality of shaping dies, which are arranged in holding tools, into the wall of the tube in a separate fabrication step before the cams are pushed on and attached.

The tool which is used for producing the depressions in the transverse direction with respect to the axis of the camshaft must be shaped in such a way that the deformation produced in the region of the wall of the tube during the pressing-in process does not result in the original external diameter of the tube being made larger in the adjacent region of the wall of the tube. This is absolutely necessary so as to permit the prefabricated cams, provided with a bore corresponding to the external diameter of the tube, to be pushed over the tube, provided with the depressions, to their position in the subsequent fabrication step.

The camshaft which is manufactured according to this method has the disadvantage that an additional, costly fabrication step for producing the depressions in the camshaft tube is necessary and that the external diameter of the tube must not be changed.

OBJECTS OF THE INVENTION

The object of the invention is therefore to specify a method for manufacturing constructed camshafts and a device for carrying out this method with which depressions can be produced on the camshaft in an easy and cost-effective way.

SUMMARY OF THE INVENTION

According to the invention, a method for manufacturing a constructed camshaft is provided, wherein a plurality of non-circumferential depressions, which are oriented parallel to the longitudinal axis and are transverse with respect to the longitudinal axis, are pressed into a tube, wherein separately prefabricated cams are attached to the tube, and wherein the pressing-in of the depressions and the attachment of the cams to the tube take place in one working step.

The cams are attached to the tube by hydroforming, with the pressing-in of the depressions taking place before the start of the increase in pressure or during the increase in pressure in the tube to a predefined final pressure.

In the device according to the invention for carrying out the method, a hydroforming tool composed of an upper tool and a lower tool with axially moveable sealing dies is used, in which hydroforming tool shaping dies, which can be moved radially with respect to the axis of the tube and as far as into the cavity formed by the closed tool, are arranged in the upper and/or lower tools.

As a result of this method, on the one hand, an additional, costly fabrication step is avoided and, on the other hand, the shaping dies can easily be integrated into the existing hydroforming tool, allowing additional tool costs to be kept very low.

According to one advantageous refinement of the invention, in a first step, before the pressing-in of the depressions, a predefined supporting pressure is applied to the tube, in which case, after this supporting pressure has been reached, the depressions are pressed in completely and the internal pressure is then increased to the final pressure to be attained.

This procedure has the advantage that it requires substantially lower die forces than if the shaping dies have to be moved counter to the maximum final pressure.

On the other hand, it is advantageous if the shaping dies are not already moved into their final position before the supporting pressure is applied so that the axial widening of the resulting deformation is reduced. Furthermore, when the depressions are pressed in without a supporting pressure in the tube, undesired deformations or buckling points may be formed, these can even be equalized by the maximum final pressure.

By optimizing the supporting pressure which is to be reached before the shaping die penetrates the tube, it is thus possible to reduce the axial widening of the depression which is formed and to avoid undesired deformations or buckling points in the tube.

Since further functional elements or additional elements such as, for example, bearing rings or gear wheels are also simultaneously joined to the tube by means of the hydroforming process, it is possible also to advantageously minimize the overall length of the camshaft by minimizing the necessary distance between the depression and the functional parts.

The maximum supporting pressures in the case of the shaping die which is drawn back in the tool should advantageously be selected such that the tube is prevented from widening into the die guide, as a result of which otherwise notches, which constitute weak points in the camshaft, may be formed.

When the depressions are pressed in, in addition to the axial movement of the sealing dies which is necessary to maintain the seal, it is advantageously possible for the tube to be pushed axially beyond this. The change in the wall thickness caused by the material flow in the region of the depression can thus be utilized to even out the thickness of the wall in particular at the depressions which are respectively closest to the end of the tube.

The final pressure is preferably selected such that the tube comes to bear against the dimension-forming bottom die and upper die contours of the hydroforming tool.

In this context, the supporting pressure and the final pressure to be reached are advantageously configured as a function of the tube material, the thickness of the wall of the tube and the diameter of the tube. A supporting pressure in the range from 200 to 1000 bar and a final pressure of 2000 to 4000 bar have proven favorable.

According to a further advantageous refinement of the method, appropriate construction of the bottom die makes it possible to ensure that the camshaft region lying radially opposite the depression and/or the camshaft regions axially adjoining the depression experience widening in comparison with the initial diameter of the tube, with the result that the bending resistance of the camshaft can be increased. The increased bending resistance thus allows larger bending torques to be transmitted than is possible according to the prior art.

The magnitude of this widening can advantageously be varied here in such a way that the functional elements or additional elements which are pushed onto the tube and formed concurrently therewith, can be protected against axial displacement on the tube by widening the tube directly next to the elements.

A further advantage according to the invention is the possibility of the freely selectable depth of the depressions. The depressions should advantageously be introduced as far as the center axis of the camshaft owing to the necessary bending strength.

In the following, the plane of intersection which is formed from the longitudinal axis of the tube and the longitudinal axis of the shaping die will be considered. The shaping die of the device according to the invention is configured in such a way that, when the shaping die moves into the tube, a desired symmetrically concave depression is produced, preferably transversely with respect to the axis of the component. For this purpose, the contour of the shaping die, which contour makes contact with the wall of the tube, has a correspondingly symmetrically convex curvature which is advantageously of semicircular design.

According to one advantageous refinement, the shaping dies which are arranged along the axis of the tube in the tool preferably have different contours. It has proven advantageous that the shaping dies located in the region of the ends of the tubes are of asymmetrically convex design. For this purpose, that side of the shaping die which faces the end of the tube has a further section of concave design starting from the convex contour. This section of concave design can optionally also be provided on both sides.

The section which is of concave design ensures that there is a rounded junction between the depression and the external diameter of the tube as the tube is subsequently pushed axially. This avoids sharp-edged junctions which may adversely affect the fatigue strength of the camshaft.

According to the invention, the shaping region of the shaping die may be modified as desired in order to produce other configurations which differ from a cylindrically concave depression.

A bead which is additionally impressed in the depression and which can preferably be arranged longitudinally with respect to the axis of the component and can also be made to penetrate more deeply, reaching beyond the center axis of the tube, proves advantageous. This bead ensures that the junction, which has extremely sharp edges in cross section, between the depression and the adjoining wall of the tube can be manufactured with a significantly enlarged radius. Reducing the notch effect in this region allows the fatigue strength to be significantly improved. The shaping die has contouring of the surface of complementary design in order to produce such a bead.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features, advantages and details of the invention emerge from the subsequent description. In the drawings:

FIG. 1 is a basic drawing of a cross section through an opened hydroforming tool with retracted shaping dies and an inserted tube and pushed-on cam rings;

FIG. 2 is a basic drawing of a cross section through a closed hydroforming tool with a tube which is fixed in position by shaping dies before the internal pressure is applied;

FIG. 3 is a basic drawing of a cross section through a closed hydroforming tool at the end of the hydroforming process with shaping dies in the final position;

FIGS. 4 a, b are basic illustrations of shaping dies according to the invention; and

FIG. 5 shows a cross section through a camshaft in the region of a depression with a bead.

DETAILED DESCRIPTION OF PREFERED EMBODIMENTS

FIG. 1 shows a tube 1 made of easily deformable material which is inserted into the opened hydroforming tool 3 which is composed of an upper tool 3 a and a lower tool 3 b. The shaping dies 2 are retracted to such an extent that the tube 1 can be inserted into the bottom die of the hydroforming tool 3 without coming into contact with the shaping dies 2. Functional elements 5 in the form of cam rings, which are connected to the tube 1 in a positionally secure fashion by means of the hydroforming process are pushed onto the tube 1.

The sealing dies 4 which are necessary for sealing the ends of the tube are already located in position between the upper tool 3 a and the lower tool 3 b but are also at an axial distance from the respective end of the tube.

The fluid supply, necessary to build up pressure, through the sealing dies 4 is not illustrated here.

After the tool 3 is closed, the sealing dies 4 come to bear against the ends 30 of the tube in a fluidtight fashion and an internal pressure of the tube is then increased to a predefined supporting pressure (FIG. 2).

After the supporting pressure has been applied, the shaping dies 2 are moved into their final position in accordance with FIG. 3 and the depressions 6 are produced on the tube 1. The internal pressure of the tube is then increased to the final pressure to be attained, with the cam rings 5 being simultaneously attached to the tube 1 in a frictionally and positively locking fashion. At the same time, the wall of the tube lying radially opposite the depression 6 and the axially adjoining regions of the tube are widened beyond the original external diameter of the tube.

FIGS. 4 a and b are examples of shaping dies according to the invention.

FIG. 4 a shows a shaping die 2 which is provided with a convex contour. Starting from a semicircular contour of the shaping die 2, the contour has, on one side, a further concave section 10 starting from the convex contour.

FIG. 4 b shows, by way of example, a shaping die 2 with a bulge S which is arranged at the apex of the contour and which produces an additional bead in the longitudinal direction of the tube when the depression is pressed in.

FIG. 5 shows a cross section through a camshaft 1 in the region of a depression 6 with an additionally impressed bead 11. The region of the junction between the depression and the semicircular wall of the tube is marked in cross section. Impressing the bead 11 ensures that the junction, which is otherwise extremely sharp-edged on the internal and external circumferences, can be manufactured with a clearly formed radius. 

1. A method for manufacturing a constructed camshaft starting from a tube into which a plurality of noncircumferential depressions, which are oriented parallel to the longitudinal axis and are transverse with respect to the longitudinal axis, are pressed, and separately prefabricated cams are attached to the tube, wherein the pressing-in of the depressions and the attachment of the cam rings to the tube take place in one working step.
 2. The method as claimed in claim 1, wherein the cam rings are attached to the tube by hydroforming, with the pressing-in of the depressions taking place before the start of or during the increase in pressure to a predefined final pressure.
 3. The method as claimed in claim 2, wherein, in a first step, a predefined supporting pressure is applied to the tube and, after this supporting pressure has been reached, the depressions are pressed in completely and the internal pressure is then increased to the final pressure to be attained.
 4. The method as claimed in claim 2, wherein, in a first step, a predefined supporting pressure is applied to the tube and, after this supporting pressure has been reached, the depressions are pressed in completely and the internal pressure is then increased to the final pressure to be attained, and wherein the final pressure is selected such that the tube comes to bear against the dimension-forming bottom die and upper die contours of the hydroforming tool.
 5. The method as claimed in claim 2, wherein the supporting pressure and the final pressure are selected in a range from 200 to 1000 bar and, respectively, in a range from 2000 to 4000 bar as a function of the tube material, the thickness of the wall of the tube and the diameter of the tube.
 6. The method as claimed in claim 2, wherein subsequent axial pushing at the ends of the tubes takes place during the rise in the internal pressure.
 7. The method as claimed in claim 2, wherein at least one of the wall of the tube lying radially opposite the depressions in the tube and the camshaft regions axially adjoining the depression are widened in comparison with the original external diameter of the tube.
 8. A device for manufacturing a constructed cam shaft, comprising a hydroforming tool which has an upper tool and a lower tool and axially movable sealing dies, wherein shaping dies, which can be moved radially with respect to the axis of the tube and as far as into the cavity formed by the closed tool, are arranged in at least one of the upper tool and the lower tool.
 9. The device as claimed in claim 8, wherein the contour of the shaping die, which contour makes contact with the wall of the tube, is curved in a symmetrically convex fashion with respect to the plane of intersection which is formed by the longitudinal axis of the tube and the longitudinal axis of the shaping die.
 10. The device as claimed in claim 9, wherein the contour of the shaping die is of convex, semicircular design.
 11. The device as claimed in claim 8, wherein the contour of the shaping die, which contour makes contact with the wall of the tube, is convexly curved in an asymmetrical fashion with respect to the plane of intersection which is formed by the longitudinal axis of the tube and the longitudinal axis of the shaping die.
 12. The device as claimed in claim 9, wherein a section which is of concave design adjoins the convexly curved contour of the shaping die on one side or on both sides.
 13. The device as claimed in claim 11, wherein a section which is of concave design adjoins the convexly curved contour of the shaping die on one side or on both sides.
 14. The device as claimed in claim 9, wherein the contour of the shaping die has a bulge formed along the axis of the tube in the region of the apex. 